Excess voltage arresters

ABSTRACT

In an excess voltage arrester comprising a pair of spaced electrodes housed in a gas-filled enclosure the electrodes extending towards one another from end caps through respective tubular insulating portions of the enclosure so that the gap between the electrodes lies within a central conductive tubular portion of the enclosure, the radial dimensions of the electrodes and the tubular portions of the enclosure are chosen so that the d.c. ignition voltage of the arrester is lower than the breakdown voltage between the electrodes and sputtered coatings which may form on the internal surfaces of the insulating portions of the enclosure.

United States Patent Baker et a1.

[ EXCESS VOLTAGE ARRESTERS [75] Inventors: Basil Offor Baker, Rickmansworth;

John William Ellis Ross, Sidcup, both of England [73] Assignee: The M-O Valve Company Limited, London, England [22] Filed: Mar. 21, 1974 [2]} Appl. No.: 453,323

[30] Foreign Application Priority Data Mar. 23, 1973 United Kingdom 14139/73 [52] U.S. Cl 317/61; 313/231.1 [51] Int. Cl. H02h 9/04 [58] Field of Search 313/214, 220, 2311; 317/61, 61.5, 62, 66, 68

[56] References Cited UNITED STATES PATENTS 3,289,027 11/1966 Jones 317/61 X 3,676,743 7/1972 Bahr et a1. .1 317/61 3,691,428 9/1972 Bahr et a1. 11 317/61 130 15c 7l5b 1521140 [4 1 May 20, 1975 3,710,191 l/1973 Peche ..3l7/61.5X 3,780,350 12/1973 Sanger et al, 317/615 FOREIGN PATENTS OR APPLICATIONS 1,205,763 9/1970 United Kingdom H 317/615 Primary Examiner-G. Harris Attorney, Agent. or Firml(irschstein, Kirschstein, Ottinger & Frank [5 7] ABSTRACT In an excess voltage arrester comprising a pair of spaced electrodes housed in a gas-filled enclosure the electrodes extending towards one another from end caps through respective tubular insulating portions of the enclosure so that the gap between the electrodes lies within a central conductive tubular portion of the enclosure, the radial dimensions of the electrodes and the tubular portions of the enclosure are chosen so that the dc. ignition voltage of the arrester is lower than the breakdown voltage between the electrodes and sputtered coatings which may form on the internal surfaces of the insulating portions of the enclosure.

4 Claims, 1 Drawing Figure PATENTED M20595 s E a. B A 1 4 Q 13. 5, N E Q 2427/ v v: a

EXCESS VOLTAGE ARRESTERS This invention relates to excess voltage arresters.

A known form of excess voltage arrester comprises a pair of spaced electrodes housed in a hollow cylindrical, gas-filled enclosure comprising electrically conductive end portions and an electrically conductive tu bular central portion which is sealed to and spaced from the end portions by respective tubular electrically insulating portions, the end portions constituting respective terminals for the two electrodes, the two electrodes extending towards each other through the insulating portions so that the gap between the electrodes lies within the central portion. Such an excess voltage arrester is hereinafter referred to as an arrester of the kind specified. One such arrester is described in UK. Patent Specification No. 1,021,782.

ln normal use of an arrester of the kind specified the two electrodes are respectively connected via their terminals to two lines, and the central portion of the enclosure is grounded. When an excess voltage appears between either one of the lines and the other line, or ground, a discharge occurs in the gas-filled enclosure, thus effectively grounding the lines.

ln an arrester of the kind specified a discharge is intended to occur between the inner end of one or both the electrodes and the central portion of the envelope, and the gap between these elements is chosen in conjunction with the pressure and composition of the gas filling to confer a desired d.c. ignition voltage on the arrester.

It has been found that with known arresters of the kind specified, after a large number of discharges have occurred, subsequent discharges tend to occur at an intermediate point along the length of one or both electrodes, at a d.c. ignition voltage lower than the designed voltage.

It is an object of the present invention to provide an arrester of the kind specified wherein this difficulty is overcome.

According to the present invention in an arrester of the kind specified, the d.c. ignition voltage across the gap between the inner end of each electrode and the adjacent part of the central portion of the enclosure is less than the breakdown voltage across any gap between a point on the surface of that electrode and a conductive coating which may be produced in operation on the internal surface of the insulating portion of the enclosure through which that electrode extends, and means are provided for restricting the cathode spot formed in operation on the inner end of either electrode to a region adjacent the inner end of that electrode.

The invention resides in the appreciation by the inventors that the failure of prior art arresters results from the formation by sputtering from the discharge of conductive coatings on the internal surfaces of the insulating portions of the enclosure. The formation of such films is enhanced in some arresters by a tendency for the cathode spot of the discharge occurring in operation to run along the surface of the electrode on which it is formed towards the end of the arrester.

In one particular arrangement in accordance with the invention to obtain the desired relation between said d.c. ignition and breakdown voltages the radial gap be tween the inner end of each electrode and the adjacent part of the central portion of the enclosure is smaller than the radial gap between that electrode and the internal surface of the insulating portion of the enclosure through which that electrode extends.

The means for restricting the cathode spot suitably comprises an electron emissive coating on the inner end of each electrode.

One arrangement in accordance with the invention will now be described, by way of example, with reference to the accompanying drawing which is a sectional view of a gas-filled excess voltage arrester.

Referring to the drawing, the arrester includes a hermetically sealed, hollow cylindrical, gas-filled enclosure comprising two cup-shaped metal end caps l and 2, a tubular metal central member 3 and two tubular ceramic members 4 and S. The ceramic members 4 and 5 are sealed at one end into the end caps 1 and 2, locating on chamfers 6, and at the other end into opposite ends of the central member 3. To this end, the outer surface of each of the ceramic members 4 and 5 is metallised in two bands 8 and 9, annular seals being established between the inter-fitting parts by brazing during assembly.

Within the gas-filled enclosure are housed a pair of axially spaced electrodes 10 and 11. The electrode 10 is formed in two parts 12 and 13 which are brazed together, end on, The outer part 12 lies mainly within the ceramic enclosure member 4 and comprises three sections 12a, 12b and 12c. The central major section 12b is a sliding fit in the ceramic member 4. The relatively short outer section 12a is of reduced external diameter and is brazed to the end cap 1. The inner section 12c is of similarly reduced external diameter, has a length about half that of the central section 12b, and at its inner end is brazed to the other part 13 of electrode 10. The part 13 also comprises three sections 13a, 13b and 13c, the section 13a adjacent section of part 12 being of smaller diameter than section 120 and of relatively short length. The central major section 13b is of the same external diameter as section 12c and has a length such that the inner section 13c, which is of relatively large external diameter, lies near the centre of the metal member 3 of the enclosure. The parts 12 and 13 have communicating axial bores 12d and 13d, the bore 12d being enlarged at its outer end to accommodate a pumping stem 23 which is pinched-off in conventional manner after the required gas-filling has been inserted in the enclosure.

The other electrode 11 is brazed to the end cap 2 at one end and extends through the ceramic member 5 so that its other end lies near to, but spaced from, the corresponding end of the electrode 10, the electrode 11 being otherwise identical with the electrode 10 except that it is not provided with a pumping stem.

ln the drawing the two parts of the electrode 11 are given reference numerals l4 and 15 with suffixes a, b and c, as for electrode 10.

The gap 16 between the electrodes 10 and 11 is made to have the same d.c. ignition voltage as the gaps l7 and 18 between the central enclosure member 3 and the innermost sections and 15(- of the electrodes.

The inner end of each electrode l0 and 11 is provided with a coating of electron emissive material (not shown).

In manufacture of the arrester each end cap 1 or 2 is first brazed to its associated electrode l0 or ll and to ceramic members 4 or 5 to form a sub-assembly, the brazing being carried out with the electrode axis vertical and the end cap lowermost, and with rings ofa suitable brazing material placed around the sections 120 and 13a of electrode 10 and the corresponding sections of electrode 11. The two sub-assemblies are then sealed into opposite ends of the central member 3 of the enclosure, and the required gas filling is inserted via the pumping stem 23.

In use of the arrester, the end caps 1 and 2 are re spectively connected to a pair of lines connected with the equipment which it is desired to protect against ex cess voltage, and the metal enclosure member 3 is grounded. On the occurrence of an excess voltage between either one of the lines and the other line, or ground, a discharge occurs between one or both electrodes 10 and 11 and the metal member 3 of the enclo sure. It will be appreciated that while initially the dis charge may occur between the two electrodes 10 and l] the discharge will rapidly transfer to the gap between one or both electrodes and the ground member 3. A desired d.c ignition voltage for the arrester, that is the voltage at which a discharge is initiated, is obtained by appropriate choice of the pressure and composition of the gas-filling and the sizes of the gaps 16, l7 and 18.

When a discharge takes place the cathode spot formed on either of the electrodes 10 or 11 is anchored to the inner end of that electrode by the presence of the electron emissive coating on that electrode. This prevents failure ofthe arrester due to the discharge travelling along the electrode and producing electrically conductive coatings of sputtered material all over the internal surfaces of the ceramic members. However, even with the cathode spots anchored, sputtered material from the discharge forms electrically conductive coatings on the internal surface of the metal member 3 and on the end faces 21 and 22 of the ceramic members 4 and 5, which coatings are electrically connected to ground in operation via the enclosure metal member 3.

In accordance with the present invention the arrester is designed so that the dc. ignition voltage across the gaps l7 and 18 is lower than the breakdown voltage across the radial gap between a conductive coating on an end face 21 or 22 and the adjacent electrode section 13b or 15b. In this way the possibility of a reduction of the dc. ignition voltage of the arrester due to the for mation of conducting coatings on the end faces 21 and 22 is prevented.

In one particular arrester of the form shown in the drawing having a dc. ignition voltage of 400 volts, the gas-filling consists of a l% hydrogen argon mixture at a pressure of 50 torr, the gap 16 is 1.5 millimetres, the gaps 17 and 18 are 1.65 millimetres, and the radial spacing between the external surfaces of the electrode sections 13b and 156 and the internal curved surfaces of ceramic members 4 and is 2.0 millimetres. With this arrangement the breakdown voltage across the radial gap between a conductive coating on an end face 21 or 22 and the adjacent electrode or 11 is about 500 volts.

In the arrester shown in the drawing the adjacent sections 13c and C of the electrodes 10 and 11 are of relatively large external diameter, but this need not be the case in an arrangement in accordance with the invention. Thus in alternative arrangements the inner ends of the electrodes may be of the same diameter as the adjacent sections of the electrodes. The required relatively low breakdown voltage across the gap between the inner ends of the electrodes and the enclosure metal member 3 may then be obtained by reducing the internal diameter of the member 3 in the region of the inner ends of the electrodes It will be appreciated that in an arrangement in accordance with the invention means other than electron emissive coatings may be employed for restricting the cathode spots to the regions adjacent the inner ends of the electrodes. One possibility is for the sections 12c and 13b electrode 10 and the sections 146 and 15b of electrode 11 to be provided with sleeves of insulating material. When such sleeves are provided, the sleeves themselves can be made effective to increase the breakdown voltage between the sputtered coatings on end faces 21 and 22 and the adjacent electrode 10 or 11 above the dc. ignition voltage of the arrester, thus eliminating the necessity for making the radial gap between the inner end of each electrode and the adjacent part of the central portion of the enclosure smaller than the radial gap between that electrode and the internal surface of the adjacent insulating portion of the enclosure to obtain a lower d.c. ignition voltage than breakdown voltage.

The provision of such sleeves forms the subject of Pa tent Application Ser. No. 453,322 filed Mar. 2|, l974 in the names of 8.0, Baker and J.W.E. Ross, which application is assigned to the same assignec as the present application,

It is pointed out that while the particular arrester shown in the drawing is designed for connection in an external circuit by means of clips or the like contacting the end caps 1 and 2 and the enclosure metal member 3, other arresters in accordance with the invention may be provided with lead wires for external connection.

We claim:

1. An excess voltage arrester comprising: a hollow cylindrical gas-filled enclosure comprising a tubular electrically conductive central portion, a pair of electrically conductive end portions, and a pair of tubular electrically insulating portions by means of which said end portions are respectively sealed to and spaced from said central portion; a pair of spaced electrodes housed within the enclosure for which said end portions serve as respective terminals, said electrodes extending towards one another through said insulating portions of the enclosure so that the gap between the electrodes lies within the central portion of the enclosure; and means for restricting the cathode spot formed in operation on the inner end of either electrode to a region adjacent the inner end of that electrode; the dimensions of the gaps, via the gas-filling, between the electrodes and the central portion of the enclosure, and between the electrodes and the internal surfaces of the insulating portions of the enclosure being such that the dc. ignition voltage across the gap between the inner end of each electrode and the adjacent part of the central portion of the enclosure is less than the breakdown voltage across any gap between a point on the surface of that electrode and a conductive coating which may be produced in operation on the internal surface of the insulating portion of the enclosure through which that electrode extends.

2. An arrester according to claim 1 wherein the radial gap between the inner end of each electrode and the adjacent part of the central portion of the enclosure is smaller than the radial gap between that electrode and portion of that electrode.

4. An arrester according to claim 1 wherein said means for restricting the cathode spot comprises an electron emissive coating on the inner end of each electrode. 

1. An excess voltage arrester comprising: a hollow cylindrical gas-filled enclosure comprising a tubular electrically conductive central portion, a pair of electrically conductive end portions, and a pair of tubular electrically insulating portions by means of which said end portions are respectively sealed to and spaced from said central portion; a pair of spaced electrodes housed within the enclosure for which said end portions serve as respective terminals, said electrodes extending towards one another through said insulating portions of the enclosure so that the gap between the electrodes lies within the central portion of the enclosure; and means for restricting the cathode spot formed in operation on the inner end of either electrode to a region adjacent the inner end of that electrode; the dimensions of the gaps, via the gas-filling, between the electrodes and the central portion of the enclosure, and between the electrodes and the internal surfaces of the insulating portions of the enclosure being such that the d.c. ignition voltage across the gap between the inner end of each electrode and the adjacent part of the central portion of the enclosure is less than the breakdown voltage across any gap between a point on the surface of that electrode and a conductive coating which may be produced in operation on the internal surface of the insulating portion of the enclosure through which that electrode extends.
 2. An arrester according to claim 1 wherein the radial gap between the inner end of each electrode and the adjacent part of the central portion of the enclosure is smaller than the radial gap between that electrode and the internal surface of the insulating portion of the enclosure through which that electrode extends.
 3. An arrester according to claim 2 wherein said central portion of the enclosure is of substantially uniform internal diameter along its length and the inner end of each electrode is of larger diameter that the adjacent portion of that electrode.
 4. An arrester according to claim 1 wherein said means for restricting the cathode spot comprises an electron emissive coating on the inner end of each electrode. 